The present invention relates to the treatment of the primary sludge that is produced from waste water treatment facilities such as a pulp mill or a pulp and paper mill, wherein cellulose fibers are a significant portion of the solid waste. It further relates to a process of using the primary sludge resulting from the waste water in the production of cellulosic ethanol. It further relates to the use of the primary sludge in the manufacture of recycled linerboard.
The manufacture of paper involves blending, in water, a pulp material (generally cellulose fiber) with fillers, such as clay, and other additives to create a stock slurry mixture referred to herein as a pulp. The pulp is then processed through a papermaking machine to form a sheet. The water is then extracted from the sheet and the sheet is then pressed and dried, thereby forming a paper product. The extracted water or industrial effluent stream, contains an amount of waste solids, which is mostly fiber and filler material.
The industrial effluent stream containing these waste solids that cannot be directly recycled by, for example, paper mill “saveall” devices, are conveyed by the sewerage system to a waste water treatment plant facility. The industrial effluent stream goes through a series of operations depending on the particular set-up of the waste water treatment facility, to concentrate and dewater the waste solids producing a sludge. Ultimately, the industrial effluent stream is passed through a filter press, wherein the waste solids are concentrated into a primary sludge and the filtered waste water from the press is further processed in an aeration pond producing a secondary or bacteria activated sludge, containing biological waste and water that is fit for discharge or reuse.
However, the largest portion of this stream is the primary sludge produced from the waste water treatment plants. After dewatering, the solids are contained in a concentrated, typically 40%-60% solids sludge. The main components of this sludge are cellulosic fibers and filler material such as clays and silicates.
As mentioned above, the filtered water after separation from the primary sludge still contains dissolved or finely suspended organic matter which needs to be reduced in order for safe discharge. This stream is further treated in an activated sludge process and sent to an aeration pond. The secondary sludge which is the product of the biological process is usually disposed of in landfills. Some papermaking processes recycle primary sludge, however, in addition to issues with strength properties, paper sludge has been found to adversely affect sizing and cause size reversion.
“Sludge” is a generic term for the solid residue that results from pulp and papermaking. Sludge is typically produced at two steps in the process of treating the effluent water from the industrial facility. Primary sludge, containing most of the suspended solids in the effluent stream, is recovered by the first stage of the processing at the primary clarifier. Primary clarification is usually carried out by sedimentation or through a filter press, but can also be performed by dissolved air flotation. In sedimentation, the waste water to be treated is pumped into large settling tanks, with the solids being removed from the tank bottom. These solids can range from 1.5% to 6.5% depending on the characteristics of the material. The overflow, or clarified water, is passed on for secondary treatment.
If the industrial effluent is passed through a filter press the primary sludge is collected and the filtered effluent piped back into the process for secondary treatment.
The waste water resulting from the separation of the primary sludge usually contains dissolved organic materials as well as some portion of finely suspended solids. These need to be reduced or eliminated before the water stream can be safely discharged or reused. Secondary treatment is usually a biological process in which micro-organisms convert the waste to carbon dioxide and water while consuming oxygen. The resulting solids are then re-moved through clarification as in the primary treatment. The resulting secondary or biological sludge is sent to settling pond. In general, primary sludge is easier to dewater than the secondary or biological sludge resulting from the second stage. This secondary or biological sludge is typically sent to a landfill.
The paper industry currently uses several methods to dispose of the sludge that pulp and paper production generates, such as landspread in the summer when the fields are accessible and incinerate for steam production during the winter when steam demand is greater. Currently, most sludge produced by pulp mills or pulp and paper mills is dewatered and landfilled. These landfills can be industrial landfills that are constructed and operated by the mills or they can be independently owned, requiring the mills to pay a “tipping fee” for sludge disposal. Current landfills are reaching capacity and new ones are difficult to site and construct because of more stringent environmental requirements.
Some alternative processes, such as fluidized bed systems, seem to be more environmentally friendly. Microbiological treatment is still relatively new and is yet to be used on a large scale. Alternative uses for sludge ash, such as bricks and cement, are an excellent option if a user can be found near the mill and if long term contracts can be acquired. New products developed from pulp and paper mill sludge, however, need to have a market to make them economically feasible. It does not make sense to develop and create products for which there is no market.
Even though there are several applications where the cellulose fiber in the primary sludge can be used for economic benefit such as for ethanol production or for recycle cellulose products, the main impediment is that the fibers in the primary sludge are quickly degraded by biological agents present naturally in the system. This reduces the value of the fiber in the sludge. In order to mitigate the biodegradation of the fibers it is desirable to add a preservative to the effluent water prior to the separation. However, the preservative will also reside in the waste water after the separation process and thus reduce the potency of the biological agents in the secondary treatment step. Thus, it is desirable to have a preservative that will substantially reduce the biodegradation of the cellulose fibers in the primary sludgy without affecting the kinetics of COD reduction in the secondary biological treatment step of the waste water. While there have been attempts to treat the sludge using various compositions and processing techniques, there is still a need to find new processes and applications wherein the primary sludge can be used as a renewable source of new products, while maintaining the efficacy of the secondary treatment step